Comparative-species and populational aspects of body mass dynamics

In this essay, two approaches to study the relationships with body mass are distinguished, one of which is comparative-species and the other is within-species, or populational. With the former approach, one deals with multi-species assemblages, and body mass is used as independent variable to build up allometric relationships. With the latter approach, body mass may be also regarded as independent variable (in studies of growth and individual development), but may - and we find it more interesting - b e regarded as dependent variable when one focuses on an animal's response, in terms of body mass, to environmental conditions. The main tool to develop comparative-species relationships is regression analysis. On the contrary, at the withinspecies level regression relationships on body mass are normally not feasible probably because the range of body mass variation is usually too narrow. Here we consider a new method, called contribution analysis, to study body mass dynamics at the population level. Based on our previous research, two examples of comparative-species and within-species relationships involving body mass are considered - the logisticregression relationship of the probability of being under threat of extinction as dependent on body mass in mammals and body mass dynamics of the cladoceran Daphnia galeata in response to food conditions, studied with contribution analysis.